The present invention relates to improvements in pipettes and, more particularly, to air displacement pipettes including a novel mounting shaft and a unique pipette tip tailored to the mounting shaft such that the tip is easily insertable by a pipette user onto the shaft to a fluid tight position in which the tip is secured against undesired lateral rocking on or displacement from the shaft and, after use, is easily ejectable from the shaft by the pipette user; such tip insertion and ejection requiring the pipette user to only exert axial tip insertion and ejection forces of about one pound or less thereby substantially eliminating all risk of repetitive motion injury to the pipette user.
The use of pipette devices for the transfer and dispensing of precise quantities of fluids in analytical systems is well known as is the use of disposable tip members for such pipettes. Disposable tips accommodate the serial use of such pipette devices in the transfer of different fluids without carryover or contamination.
Generally speaking, disposable pipette tips are formed of a plastic material and are of a hollow, elongated, generally conical shape with an open proximal end for receiving and releasably mating with the distal end of an elongated generally conical pipette tip mounting shaft of a pipette device. Ideally, the disposable tip should slide easily onto the mounting shaft to an axial position adjacent a lower end of a tip ejection mechanism of the pipette device. Thus located, the pipette tip should be laterally stable on the shaft, free from external rocking relative to the shaft (as during "touching off"), and should form a fluid tight annular seal with the mounting shaft. Then when it is desired to replace the tip with a new tip, the pipette tip should be easily removed from the mounting shaft by operation of the tip ejection mechanism.
To meet the desired sealing criteria for disposable pipette tips on pipette tip mounting shafts, the inner surface and side walls of the proximal portions of most pipette tips are axially tapered at a one to one and a half degree greater angle than the distal end of the pipette tip mounting shaft and form an axially elongated frusto-conical annular sealing band. The sealing band is dimensioned to stretch outwardly ("hoop stretch") as the distal end of the elongated generally conical pipette tip mounting shaft is forced into the proximal end of the tip to firmly seat the tip on the shaft and to create an axially elongated annular fluid tight seal between the sealing band and the mounting shaft. Other pipette tips, such as those described in U.S. Pat. Nos. 4,748,859 and 4,824,641, include a plurality of axially spaced compressible annular sealing rings on an inner surface of the proximal end portion of such tips. The rings create multiple axially spaced fluid tight annular seals between the outer surface of the pipette tip mounting shaft and the inner surface of the proximal end portion of the tip which by virtue of the axially spaced rings is laterally stabilized against undesired rocking on the shaft during touching off.
Usually, in mounting a pipette tip on a mounting shaft of a pipette, a user, exerting a downward force of between twelve and fifteen pounds, drives the mounting shaft axially into the tip a distance which to the user seems sufficient to create (i) a fluid tight seal between the tip and (ii) the desired lateral stability for the tip on the shaft. On occasion, in a mistaken attempt to improve the lateral stability of a pipette tip on a mounting shaft, a user will exert a downward insertion force (e.g. eighteen to twenty-five pounds) on the shaft sufficient to axially drive the tip on the shaft until an upper surface of the tip engages or is wedged into the ejector arm or cone of the tip ejector mechanism of the pipette. The contact between a lower surface of the tip ejector arm or cone and the upper surface of the tip, however, only provides a minimal resistance to rocking of the tip on the shaft and hence only results in a minimal increase in the lateral stability of the tip on the shaft. Further, since most pipette tips are formed of a relatively rigid plastic material, the annular stretching of the pipette tip required to accommodate movement of the tip onto the shaft particularly to a point where it engages the lower surface of the tip ejector or cone is difficult to achieve. In fact, the axial forces which must be exerted on a conventional pipette to achieve such a positioning of the tip on the pipette tip mounting shaft exceed twelve and may be as great as twenty pounds, which is difficult for many pipette tip users to generate. Of course, with most pipette tip designs, the greater the axial force exerted in seating a pipette tip on a pipette mounting shaft, the greater the force required to eject the tip from the mounting shaft. Thus, while the insertion of a pipette tip onto a mounting shaft until it reaches a position against a lower surface of a pipette tip ejector mechanism provides a minimum increase in the lateral stability of the tip on the shaft, it works against the design criteria for disposable pipette tips that they be easily removable from the shaft when it is desired to replace the tip.
In fact, the design criteria for disposable pipette tips that they be stably mountable on and form a fluid tight seal with a pipette mounting shaft is more easily achieved than the design criteria that disposable pipette tips slide easily onto a pipette tip mounting shaft to an axial location forming a fluid tight seal and then be easily removable from the mounting shaft when it is desired to replace the tip.
In these regards, the pipette tip mounting shafts of devices for pipetting volumes of liquid in different ranges have different external shape. For example, the distal end of standard pipette tip mounting shafts of pipettes for pipetting liquids in volumes greater than 500 microliters (large volume pipettes) commonly have a downward and inward axial taper of about one and one half to two and one half degrees per side from the longitudinal axis of the mounting shaft. On the other hand, the distal end of the mounting shafts of moderate to relatively small volume pipette devices (250 microliters and less) commonly have a downward and inward axial taper of about two to five degrees per side from the longitudinal axis of the mounting shaft so that the nose of the shaft will hit the inner wall of the pipette tip and cause hoop stretching thereof before the side of the shaft engages the inner wall of the tip. Therefore, while the design criteria that a large volume pipette tip be easily mountable on and easily removable from the mounting shaft of a large volume pipette device may be achieved by including a proximal end portion having a side wall of reduced wall thickness as in the large volume pipette tip described in U.S. Pat. No. 5,779,984, issued Jul. 14, 1998, such a thin wall design will not result in a pipette tip that satisfies the easy mount and ejection design criteria of moderate and small volume pipette tips which must firmly mount on pipette tip mounting shafts having an inward taper of two degrees and above. The same is true of the pipette tip design disclosed in U.S. Pat. No. 4,072,330 which includes a frusto-conical sealing region having a thin side wall.
As previously stated, standard small and moderate volume pipette tips include a frusto-conical annular sealing band or inner surface for engaging and sealing with the tapered distal end of a pipette tip mounting shaft. The angle of taper of the sealing surface usually approximates (e.g. one and one-half degrees greater than) that of the mounting shaft (e.g. two to five degrees). Thinning the side wall of the standard small and moderate volume pipette tips in the region of such a sealing band does little to reduce the mounting and ejection forces required to move such a tip to a sealing location and then eject the pipette tip from the mounting shaft. In forming the desired annular seal, the frusto-conical annular region is required to stretch like a hoop (hoop stretch) outwardly normal to the mating sloping surface of the pipette tip mounting shaft. Large reactive forces in the tip material resist such hoop stretching and require the exertion of large axial forces (eg. ten or more pounds) on the tip in order to mount the tip on the mounting shaft and create the necessary annular fluid tight seal. Such reactive forces increase as the tip is driven toward the tip ejection mechanism of the associated pipette device.
Further, disposable pipette tips are commonly mounted and stored in sterilizable racks. Such racks commonly include a support tray having an array of holes for receiving distal ends of pipette tips to vertically orient the pipette tips in a spaced rectilinear pattern with open proximal ends of the tips exposed to receive the mounting shafts of a pipette device onto which the pipette tips are to be mounted. For example, to mount the disposable pipette tips contained in a tip rack on the shafts of a multi-channel pipette, the pipette device is placed over the rack with its several mounting shafts aligned with the open proximal ends of an aligned series of the pipette tips. After a slight initial insertion of the mounting shafts into the open proximal ends of the aligned pipette tips, a relatively large downward force is exerted on the pipette device to drive the mounting shafts into the tip members. The pipette tips are thus very firmly seated on the mounting shafts and are lifted from the rack with upward movement of the multi-channel pipette. Unfortunately, in practice, such multiple pipette tip mounting procedures often result in some of the pipette tips being mounted at different axial locations on some of the mounting shafts. In an attempt to eliminate such non-uniform mounting of pipette tips on the several channels of a multi channel pipette, users often rock the pipette as the mounting shafts are driven by axial forces approximating 12 to 15 pound per channel into the tips supported by a pipette tip rack to drive the tips toward the lower surface of the tip ejector mechanism of the pipette.
Moreover, the more firmly a tip is mounted or wedged on the mounting shaft of the pipette device, the greater the axial force which a pipette user must generate by thumb and hand action to eject the tip from the shaft when a tip replacement is desired. In practice, it is not uncommon for axial forces approximating ten pounds per pipette channel to be generated by the pipette users thumb and hand in driving a tip from a mounting shaft. Over several and repeated ejection operations, particularly with multi-channel pipettes where the generation of substantially greater axial forces is required, the thumb and hand of the user become physically stressed often resulting in repetitive stress injury to the thumb and hand and in extreme cases, carpal tunnel syndrome.
Still further, standard pipette tips as well as those illustrated in U.S. Pat. No. 4,072,330 depend solely upon the sealing region of the pipette tip to both create the annular fluid tight seal and to provide the stable lateral mounting of the tip to the shaft sufficient to resist rocking as during touching off. The structure of such pipette tips do not provide such lateral mounting stability and but for those rare instances where the tips are jammed upward against the bottom of the pipette tip ejector arm or cone, minimal lateral stability of the tip on the shaft is achieved.
In an effort to improve lateral stability and retention of pipette tips on the mounting shafts of some pipettes, some manufacturers include O-rings on and encircling the tip mounting shafts of their pipettes. For example, the Brinkmann Instrument Co. indicates for its Transferpipette 8/12 that such O-rings ensure that all tips stay firmly mounted during use. However, there is a rapid wearing of such O-rings with repeated insertion of the associated mounting shafts into and ejection of pipette tips from such shafts. With such wear, the tips no longer stay firmly mounted during use and wear particles from the O-rings can contaminate fluid samples handled by the associated pipettes.
In an effort to reduce the hand and finger forces which a pipette user must generate to eject a tip from the mounting shaft of a pipette, other pipette manufacturers such as LabSystems have developed and include in some of their pipettes gear and ratchet mechanisms for amplifying the user generated forces to eject pipette tips from their mounting shafts. Unfortunately, such mechanisms are costly and add undesired size and weight to the pipettes.
More recently, to meet the previously described ideal characteristics and criteria for a pipette tip, there has been developed an improved plastic pipette tip which is mountable on and ejectable from a standard pipette mounting shaft of an air displacement pipette by application of an axial mounting force of less than six pounds and an axial ejection force as small as three pounds. The improved pipette tip is described in the concurrently filed U.S. patent application, Ser. No. 09/188,030, entitled "Easy Eject Pipette Tip". As there described, to meet the mountability and ease of ejection criteria for disposable pipette tips, the improved pipette tip, hereinafter referred to as the "Soft Seal" tip, includes an open tubular proximal end portion comprising an enlarged frusto-conical open top tapering downwardly and inwardly to join at an annular sealing region to a hollow substantially cylindrical mid-portion of the pipette tip. The open top has an inner diameter sufficient to axially receive the distal end of a standard pipette tip mounting shaft. The annular sealing region is formed by the transition or line of connection of the frusto-conical open top to the mid-portion of the pipette and includes an annular sidewall having a thickness in a range of 0.20 to 0.50 mm. The mid-portion has an inner diameter at the sealing region which is less than the diameter of the pipette mounting shaft, a thin resilient annular side wall having a thickness in a range of 0.20 to 0.50 mm and an axial length in a range of 0.25 to 0.65 cm. Thus, while the distal end of the mounting shaft fits into the enlarged open end of the pipette tip, the frusto-conical outer surface of the mounting shaft engages the inner surface of the sealing region at the bottom of the open top of the pipette tip to stretch the annular sealing region or line radially outward as the mounting shaft is inserted into the proximal portion, thereby creating a fluid tight seal between the sealing zone and the sealing region. In addition to the proximal portion, the improved pipette tip includes a tubular distal portion extending from the mid-portion and terminating in a relatively narrow distal end opening for passing fluid into and from the tip upon operation of the pipette device. Finally, the improved pipette tip preferably includes lateral stabilizing means on its inner surface adjacent the sealing region for engaging the outer surface of the mounting shaft as it is inserted into the proximal portion to laterally stabilize the tip on the shaft. Such lateral stabilizing means preferably comprises at least three circumferentially spaced contacts extending inwardly from the inner surface of the proximal portion of the tip adjacent the sealing region for engaging the outer surface of the mounting shaft as it is inserted into the proximal portion to laterally stabilize the tip on the shaft. In this regard, the diametric spacing of the contacts is such that the contacts lightly engage and allow the distal end of the shaft to pass with no hoop stretching of the sidewalls from which the contacts extend. In this manner, the contacts combine with the sealing region to provide lateral support for the pipette tip on the mounting shaft and prevent the pipette tip from moving laterally when lateral external forces are exerted on the distal portion of the tip as during touching off.
While the improved pipette tip as described above represents a substantial improvement over standard pipette tips with respect to the axial forces which are required to mount the tip on and eject the tip from a pipette mounting shaft, there is a continuing need to still further reduce the risk of repetitive motion injuries to pipette users and a continuing desire to further minimize the axial forces which are required to stably mount a pipette tip on and eject a pipette tip from a pipette mounting shaft. The present invention satisfies that need.